1. Field of Invention
The present invention relates to a DC/AC inverter, and more particularly to a DC/AC inverter for driving a background fluorescent lamp of an LCD, especially for a plurality of power operated half-bridge DC/AC inverter designed for driving a plurality of fluorescent lamps.
2. Description of Related Arts
As liquid crystal displays (LCD) thinner than conventional cathode ray tube (CRT) monitors, they are being used in more and more homes and public display. However, since LCD is operated by its optical rotary power and optical characteristic to display image and text information, hence not illuminable, it requires an additional backlight source. An example of backlight source for LCD is fluorescent lamps.
A typical DC/AC inverter utilizes a full-bridge inverter circuit, along with a resonant tank and a step-up transformer, a DC power input can be filtered and converted to a high AC voltage by the resonant tank and the step-up transformer, so as to drive the fluorescent lamp.
In order to stabilize the light emitted by the fluorescent lamp, and prevent a change in light intensity while there is a change in the power input voltage, most inverters are incorporated with negative feedback circuits for stabilizing the current in the fluorescent lamp. As the life-span of the fluorescent lamp is affected by the symmetry of the waveform of the current, it is most popular to use full-bridge inverter to drive fluorescent lamps.
Referring to FIG. 1 of the drawing, a conventional full-bridge inverter is illustrated. As shown in FIG. 1, a full-bridge inverter 100 comprises a DC voltage source 101, a full-bridge switch circuitry 102, a resonant tank 103, a fluorescent lamp 104, a current sensing circuit 105, a pulse width modulator 106, a frequency generator 107, and a full-bridge switch driver circuit 108, wherein the full-bridge switch circuitry 102 comprises four power switches 101A, 101B, 101C and 101D. The resonant tank 103 comprises a step-up transformer 120 and two resonant capacitors 121 and 122. The frequency generator 107 comprises a triangular wave generator 110 and a pulse generator 109. The full-bridge switch driver circuit 108 provides four sets of driving signal R1, R2, R3 and R4.
The DC voltage source 101 is electrically connected to the full-bridge switch circuitry 102, wherein the output of the full-bridge switch circuitry 102 is electrically connected to an input of the resonant tank 103. An output of the resonant tank 103 is electrically connected to a terminal of the fluorescent lamp 104. The series connection between the full-bridge switch circuitry 102, the resonant tank 103 and the fluorescent lamp 104 is a typical example of a power transfer connection.
The current sensing circuit 105 is electrically connected to the fluorescent lamp 104 and the pulse width modulator 106. The pulse width modulator 106 is then electrically connected to the frequency generator 107 and the full-bridge switch driver circuit 108, which is electrically connected to the gate terminals of the full-bridge switch circuitry 102, forming a control loop connection.
Conventional full-bridge inverter is operated based on the a fixed high frequency conduction between the four power switches of the full-bridge switch circuitry 102, such that the DC voltage output by the DC voltage source 101 is transformed to and outputted as a fixed high-frequency AC square wave, which is provided for being inputted to the resonant tank 103. The resonant tank 103 utilizes the step-up characteristic and the filter function of the step-up transformer 120 to transform the fixed high-frequency AC square wave to a fixed high frequency AC sine wave, which is provided to the fluorescent lamp 104.
The control loop utilizes the current sensing circuit 105 to produce a feedback signal R5, which corresponds to a fluorescent lamp current, which is then transferred to the pulse width modulator 106. The pulse width modulator 106, together with the a triangular wave output R6 by the triangular wave generator 110 of the frequency generator 107, utilizing the theory of negative feedback, produces a pulse width modulation signal R7 for inputting to the full-bridge switch driver circuit 108, wherein the full-bridge switch driver circuit 108 utilizes the pulse width modulation signal R7 and the frequency generator 107 to produce the four sets of driving signals R1, R2, R3 and R4 so as to drive the four power switches 101A, 101B, 101C and 101D.
By controlling the conduction period between the two power switches 101A and 101D, and the conduction period between the two power switches 101C and 101B, the alternating conduction between 101A and 101D, and 101C and 101B provides a stable fluorescent lamp current which is an AC current having a symmetrical waveform.
This conventional type of full-bridge inverter circuits can stably control the current of a fluorescent lamp, however, has the draw back of having a great number of switches, pushing the production cost of such circuits higher.
As a result, the present invention is to provide a cheaper and more reliable DC/AC inverter.